Keywords

1 Introduction

Uranium refining and conversion is the process of further removing impurities and neutron poisons from natural uranium ore concentrate and making uranium fluoride reach nuclear grade. Wet uranium refining and conversion process were used in the majority of countries in the world. First, the uranium concentrate is dissolved by nitric acid, and then TBP-kerosene extraction method is used in uranium refining, the loaded solvent can be stripped by hot water (trace nitric acid) or ammonium carbonate solution. Finally, the loaded strip is prepared into uranium fluoride. The core purification process of wet uranium refining and conversion is TBP-kerosene/UO2(NO3)-HNO3 extraction.

Compared with other extraction processes, uranium refining extraction has its own characteristics: high concentration of target components and especially low concentration of impurities. In order to to reduce the extraction of impurities and achieve a better purification effect, maintaining a higher extraction saturation of uranium is necessary in industry. However, it is difficult to stabilize the operation for negative correlation between high extraction saturation and low uranium concentration in raffinate. In this paper, a saturated re-extraction technology based on fractionation extraction is proposed to solve this problem.

Fractionation extraction is an important extraction technology. Compared with the conventional multistage countercurrent (or continuous countercurrent) solvent extraction, fractionation extraction add one or more inlet or outlet. This method is suitable for the extraction of two or more components which is difficult to separate. Lots of research of fractionation extraction were shown in other industries [6–12]. Scrub process with loaded organic phase is necessary for further improving the effect of refining in uranium refining extraction. Extraction – scrub can also be regarded as fractionation extraction if scrub raffinate is incorporated into the feed solution. Only extraction process is discussed in this paper, not including scrub.

2 Theory of Fractionation Extraction

The saturated re-extraction process proposed in this paper is as follows: after extracted and reextracted through N-level (or continuous) counter-current operation successively, the stripped solvent become the loaded organic phase of re-extraction, and then the loaded solvent is recycled by scrubbing and stripping. A certain concentration of feed solution is prepared for the first stage of extraction and re-extraction respectively. The reextracted raffinate (N’ raffinate) is returned to prepare the feed solution. The process is shown in Fig. 1.

Fig. 1.
figure 1

Process flow chart

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The symbols of each part are as follows:

QO——Flow rate of organic phase

QA——Flow rate of the extraction feed solution

QA\({^{\prime}}\)——Flow rate of the reextraction feed solution

QF0——Flow rate of new feed fluid (or imaginary new feed fluid)

x0——Concentration of the new feed solution (or imaginary new feed solution)

xF——Concentration of the extraction and reextraction feed solution

y0——Concentration of stripped solvent

y1——Concentration of loaded solvent after extraction

y1\({^{\prime}}\)——Concentration of loaded solvent after reextraction

\(\xi\)——Saturation of loaded solvent after extraction

\(\xi {^{\prime}}\)——Saturation of loaded solvent after reextraction

N——Stage of extraction

\(N{^{\prime}}\)——Stage of reextraction

xi——Concentration of i# grade aqueous in extraction

yi——Concentration of i# grade organic phase in extraction

xj\({^{\prime}}\)——Concentration of j# grade aqueous phase in extraction

yj\({^{\prime}}\)——Concentration of j# grade aqueous phase in reextraction

xN——Concentration of extraction raffinate

xN\({^{\prime}}\)——Concentration of reextraction raffinate

n——Extraction flow ratio, n = QO /QA

n\({^{\prime}}\)——Reextraction flow ratio, n\({^{\prime}}\)= QO /QA\({^{\prime}}\)

The flow rate of reextraction aqueous phase is greater than that of extraction, that means QA\({^{\prime}}\) >QA, or n\({^{\prime}}\)< n. n takes a relatively large parameter so that the uranium concentration of raffinate can easily reach a lower standard in the extraction part. While, n’ tend to be taken as a relatively small parameter to improve the saturation of the loaded solvent in the reextraction part.

3 Results and Discussion

3.1 Extraction Equilibrium Isotherms

Uranium refining extraction is a double-component extraction of nitric acid and uranyl nitrate from the point of view of constant concentration, and extraction equations are as follows:

$${\text{UO}}_{2}{{^{2} }} _{{\left( {\text{w}} \right)}} + 2{\text{NO}}_{2}{^{ -} } _{{\left( {\text{w}} \right)}} + 2{\text{TBP}}_{{\text{o}}} \rightleftharpoons {\text{UO}}_{2} ({\rm{NO}}_{3} )_{2} \cdot2{\text{TBP}}_{{\text{o}}}$$
$${\text{H}}^{ + } _{{\left( {\text{w}} \right)}} + {\text{NO}}_{3}{^{ -} } _{{\left( {\text{w}} \right)}} + {\text{TBP}}_{{\left( {\text{o}} \right)}} \rightleftharpoons {\text{HNO}}_{3} \cdot {\text{TBP}}_{{\left( {\text{o}} \right)}}$$

The equations for the equilibrium constants are as follows:

$$K_{{\text{U}}} = \frac{{\{ {\text{UO}}_{2} ({\text{NO}}_{3} )_{2} \cdot 2{\text{TBP}}\} }}{{\left\{ {{\rm{UO}}_{2}^{{2 + }} } \right\} \cdot \{ {\text{NO}}_{3}^{ - } \} ^{2} \cdot \{ {\text{TBP}}\} ^{2} }} = \frac{{y_{{\text{U}}} \gamma _{{\text{U}}} }}{{x_{{\text{U}}} \cdot ({\text{NO}}_{3}^{ - } )^{2} \cdot \gamma _{ \pm }^{3} \cdot T^{2} \cdot \gamma _{T}^{2} }}$$
$$K_{{\text{H}}} = \frac{{\left\{ {{\text{HNO}}_{3} \cdot {\text{TBP}}} \right\}}}{{\{ {\rm{H}}^{ + } \} \left\{ {{\text{NO}}_{3}^{ - } } \right\}\left\{ {{\text{TBP}}} \right\}}} = \frac{{y_{{\text{H}}} y_{{\text{H}}}^{'} }}{{x_{H} \cdot \left( {{\text{NO}}_{3}^{ - } } \right) \cdot \gamma _{ \pm }{^{'}} ^{2} \cdot T \cdot \gamma _{T} }}$$

The activity coefficient can be calculated:

$$\begin{array}{*{20}{l}} {y}_{\rm{U}}=\overline{{K }_{\rm{U}}}{x}_{\rm{U}}{\left({\rm{NO}}_{3}^{-}\right)}^{2}{T}^{2}{\gamma }_{\pm }^{3}\\{y}_{\rm{H}}=\stackrel{\sim }{{K}_{\rm{H}}}{x}_{\rm{H}}\left({\rm{NO}}_{3}^{-}\right)T\\F=\frac{{f}_{\rm{U}}}{{(1+{f}_{\rm{H}})}^{2}}\\{f}_{\rm{U}}=\overline{{K }_{\rm{U}}}{x}_{\rm{U}}{\left(2{x}_{\rm{U}}+{x}_{\rm{H}}\right)}^{2}{\gamma }_{\pm }^{3}\\{f}_{\rm{H}}=\stackrel{\sim }{{K}_{\rm{H}}}{x}_{\rm{H}}\left(2{x}_{\rm{U}}+{x}_{\rm{H}}\right)\\{y}_{\rm{U}}=\frac{1}{2}[{T}_{0}-\frac{1}{4F}\left(\sqrt{1+8F{T}_{0}}-1\right)\end{array}$$
(1)
$${y}_{\rm{H}}=\frac{{f}_{\rm{H}}}{1+{f}_{\rm{H}}}({T}_{0}-2{y}_{\rm{U}})$$
(2)

\({y}_{\text{U}}\)— Concentration of UO2(NO3)2. 2TBP in organic phase at equilibrium (mol/L)

\({x}_{\text{U}}\)— Concentration of UO22+ in aqueous phase at equilibrium (mol/L)

\({y}_{H}\)— Concentration of HNO3.TBP in organic phase at equilibrium (mol/L).

\({x}_{H}\)— Concentration of HNO3 in aqueous phase at equilibrium (mol/L).

\(T\)— Concentration of free TBP at equilibrium (mol/L).

\({T}_{0}\)— Concentration of total TBP (mol/L).

\({\gamma }_{\pm }\)— Activity ionic activity coefficients of UO22+ and NO3− in aqueous phase.

\({\gamma }_{\text{U}}\)— Activity coefficient of UO2(NO3)2. 2TBP in organic phase.

\({\gamma }_{\pm }^{^{\prime}}\)— Average ionic activity coefficients of H+ and NO3− in aqueous phase.

\({\gamma }_{\text{H}}^{{^{\prime}}}\)— Activity coefficient of HNO3. TBP in organic phase.

\({\gamma }_{T}\)— Activity coefficient of TBP.

\(\overline{{K }_{\text{U}}}=63\)— The equivalent equilibrium constant of uranium.

\(\stackrel{\sim }{{K}_{\text{H}}}=0.19\)— The apparent equilibrium constant of hydrogen ions.

A.M.Pozeh calculated the effect on γ± according to Harend rule in the presence of nitric acid and other nitrates in aqueous solution. In uranyl nitrate and nitric acid system, xEQ = (xU + xH/3). According to the data fitting in  Fig. 2 the relationship can be obtained as Eq. (3). According to Eq. (1), (2) and (3), the extraction equilibrium line for uranium with different nitric acid concentration can be calculated under the condition that 30%TBP- kerosene is used as extraction agent (Fig. 3).

Fig. 2.
figure 2

The relationship between average ionic activity coefficients γ± and component concentration

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$${\gamma }_{\pm }=0.0317{{x}_{\rm{EQ}}}^{3}+0.2951{{x}_{\rm{EQ}}}^{2}-0.0983{x}_{\rm{EQ}}+0.539$$
(3)
Fig. 3.
figure 3

Isotherms of uranium extraction at different nitric acid concentrations

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3.2 Discussion of Operation Step Line

Operation line of reextraction process is shown in Fig. 4.

Fig. 4.
figure 4

Operation line of reextraction process

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In the figure, SA is the extraction line and CB is the reextraction line. The organic phase is extracted from point S to point A, and then reextracted from point C to point B. The reextraction feed solution is reextracted from point B to point C and becomes reextraction raffinate. The extraction feed solution is extracted from point A to point S and becomes raffinate. SD is an imaginary operating line when extraction-reextraction is considered as a whole. The operating line does not represent the actual internal state of the actual operation, but can represent the state of the feed solution in the import and export. The operating line is very useful for the research of reextraction process. The relationship between the length of segment and flow ratio in in reextraction n’, flow ratio in extraction n, component concentration and flow rate in the Fig. 4 is shown as follows.

$$\begin{array}{*{20}{l}} \overline{\text{CA}}={x}_{\text{F}}-{x}_{N}^{^{\prime}}=\frac{{Q}_{\text{O}}}{{Q}_{A}{^{\prime}}}\left({y}_{1}{^{\prime}}-{y}_{1}\right)=\frac{{Q}_{\text{O}}}{{Q}_{A}{^{\prime}}}\overline{\text{BA}}\\\overline{\text{EA}}={x}_{0}-{x}_{\text{F}}=\frac{{Q}_{\text{O}}}{{Q}_{A}}\left({y}_{1}{^{\prime}}-{y}_{1}\right)=\frac{{Q}_{\text{O}}}{{Q}_{A}}\overline{\text{DE}}\\\frac{\overline{\text{CA}}}{\overline{\text{EA}}}=\frac{{x}_{\text{F}}-{x}_{N}^{{^{\prime}}}}{{x}_{0}-{x}_{\text{F}}}=\frac{{Q}_{\text{A}}}{{Q}_{A}{^{\prime}}}=\frac{n{^{\prime}}}{\text{n}} \end{array} $$
(4)

Formula (4) shows the internal relationship between concentration and flow rate caused by the solid-liquid cycle of extraction-reextraction, and it is an important basis for regulating flow ratio and concentration of extract feed solution. The feed solution with 3.5 mol/L nitric acid and 300 g/L uranium is generally used in uranium refining extraction in industry for better effect. Uranium refining extraction is a double-component extraction of nitric acid and uranyl nitrate from the point of view of constant concentration, and the concentrations of acid and uranium at all stages of the multistage countercurrent process are changed. In this study, the uranium extraction equilibrium line with 3.5mol/L nitric acid was used as the basis of the operation line research. Operation line and step diagram are shown in Fig. 5 and Table 1 according to different operating conditions.

Fig. 5.
figure 5

Operation line and step diagram under different conditions

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Table 1. Countercurrent progression required under different operating conditions
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Figure 5 shows that using extraction-reextraction with N’ = 1–2 can improve extraction saturation greatly. Reextraction technology hardly increase the extraction stages compared with the common countercurrent extraction technology. The equipment stage efficiency has a great impact on extraction stage compared with the change of concentration conditions. The inverse of the flow ratio of uranium refining is equal to the slope of the operation line k≈y1/xF. Only if the slope of the operation line controls within the range of (124–120) /120 = 3% can the concentration of the organic phase reach to 120–124 g/L. Otherwise, the concentration of the organic phase cannot meet the requirements, or the concentration of raffinate exceeds the standard. The operating conditions are extremely rigor. This problem does not exist in reextraction technology. The control accuracy range of the extraction section can be preset at about 20%, such as (124–100) /100 = 24% or (124–108) /108 = 15%, and then the reextraction operation without special control accuracy can be carried out.

3.3 Continuous Operation and Stability Control Investigation

The feed solution was prepared as 240g/L for extraction-reextraction experiment. Extraction-reextraction operation line and step diagram are shown in Fig. 6. xF = 240g/L, y1\({^{\prime}}\)=120 g/L, xN < 0.05 g/L, y0 = 0 g/L, extraction flow ratio n = 3, reextraction flow ratio n \({^{\prime}}\)=1, stage efficiency is 80%. The figure clarifies that N = 6, N’ = 2.

Fig. 6.
figure 6

Operating line and step diagram under test conditions

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6-stage extraction and 2-stage reextraction operation was used in the experiment which was run continuously for 81h. After stable operation, the concentration of uranium in extraction loaded solvent (y1), the concentration of uranium in reextraction loaded solvent (y1\({^{\prime}}\)) and the concentration of uranium in raffinate (xN) at different times were obtained. The result is shown in Fig. 7.

Fig. 7.
figure 7

The concentration of uranium at different times

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Figure 7 clarifies that saturation reextraction technique had a good performance in the stable operation control of uranium refining. The concentration of uranium in raffinate keeps low during the whole extraction process, xN is lower than 10 mg/L. the saturated and reextraction organic phase concentration The average of y1\({^{\prime}}\) was 124 g/L, which was greater than 120 g/L. The saturation of reextraction loaded solvent ξ’ reached more than 96.7%. The concentration of uranium in export is stable. In addition, the concentration of uranium in extraction loaded solvent fluctuates between 80 and 100 g/L, which indicate that the control conditions of extraction part was not accurate. However, the concentration of export organic phase can be rapidly increased and kept stable by using saturation reextraction.

3.4 Purification Effect

The concentration of uranium and impurity components before and after extraction are shown in Table 2. It indicates that the impurity content of the loaded solvent was greatly reduced compared with the feed solution, and a good purification effect was achieved after saturation reextraction.

Table 2. Concentration of uranium and impurity components before and after extraction
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4 Conclusion

TBP extraction is the core purification process of the wet method for uranium refining and conversion. In order to achieve better refining results, saturation of uranium extraction usually keeps high in refining. However, high extraction saturation is not easy to achieve. High extraction saturation may lead to the excess concentration of uranium in raffinate. In this study, reextraction for uranium refining based on fractionation extraction was proposed according to the characteristics and requirements of uranium refining extraction. The law of reextraction is obtained. 1 ~ 2 stages reextraction operation after extraction are generally used to achieve higher extraction saturation. Reextraction technology hardly increase the extraction stages compared with the common countercurrent extraction technology. The re-extraction bench test shows that the re-extraction process can run stably. After extraction, the concentration of uranium in organic phase can reach 120g/L, the extraction saturation can reach 96.7%, the concentration of uranium in raffinate is less than 10mg/L, and the concentration of impurity components is greatly reduced. Reextraction technology can facility the control of uranium refining extraction, which maintains the state of high saturation vs. low raffinate concentration.